Fix final round of gcc warnings relating to unused parameters.

* debug.cc (iscygthread): New function.
* debug.h: Declare it.
* exceptions.cc (set_process_mask): Flush pending signals.
(handle_sigsuspend): No need to flush pending signals.
(call_handler): Refine previous tests of muto ownership.  Only clear wait()'s
when we have definitely responded to a signal.
* fhandler_console.cc (fhandler_console::read): Don't set EINTR if executing in
a "cygwin" thread.
* sigproc.cc (proc_subproc): Use second argument to control whether CLEARWAIT
actually sets "signalled" flag.
* sync.h (muto): Add 'unstable' method.

winsup/cygwin/sync.cc

@@ -103,11 +103,11 @@ muto::release ()
   /* FIXME: Need to check that other thread has not exited, too. */
   if (!--visits)
     {
+      tid = 0;		/* We were the last unlocker. */
       InterlockedExchange (&sync, 0); /* Reset trigger. */
       /* This thread had incremented waiters but had never decremented it.
 	 Decrement it now.  If it is >= 0 then there are possibly other
 	 threads waiting for the lock, so trigger bruteforce. */
-      tid = 0;		/* We were the last unlocker. */
       if (InterlockedDecrement (&waiters) >= 0)
 	(void) SetEvent (bruteforce); /* Wake up one of the waiting threads */
     }